1. Field of the Invention
The present invention relates to a grating-interference type displacement meter, and more specifically to that capable of assuring stable interference even when a scale has a uneven surface.
2. Description of the Prior Art
A photoelectric encoder is well known heretofore, which includes a scale on which optical graduations are formed with a given pitch to generate a periodic detection signal. The photoelectric encoder has its resolution defined by the width of a groove of an optical grating and a pitch which is a distance between adjacent grooves of the grating, and defined by characteristics of an electronic-circuit for processing a signal after photoelectric conversion. Such an optical grating is generally formed by etching and hence has the atmost resolution of approximately 4 micro meter in view of final measurement accuracy, and finally practical resolution of approximately 1 micro meter if the electronic circuit is assumed to be used without costing up severely. It is therefore difficult to provide a further accurate optical grating.
In contrast, with the spread of a photoelectric type encoder, it is increasingly required to generate a detection signal at high resolution and with high accuracy.
To further improve the resolution of such a photoelectric type encoder, a grating interference type displacement meter has been proposed, in which fine pitch (typically about 1 micro meter) graduations are formed on a scale by holography and used as a diffraction grating to positively produce diffraction thereon for obtaining a detection signal.
Referring to FIG. 10, a conventional grating interference type displacement meter as disclosed in Japanese Laid-Open Publication No. 47-10034 is illustrated. The grating-interference type displacement meter includes a scale, on which a diffraction grating 10A of a pitch d has been formed, a He - Ne laser light source 12 for emitting a laser beam 14 of a wavelength .lambda. as an optical flux to irradiate the diffraction grating 10A therewith, mirrors 16, 18 for reflecting zeroth-and first-order diffracted optical beams produced by the diffraction grating 10A, respectively, a beam splitter (coarse diffraction grating) 20 for splitting into three equal optical beams a combined beam of a zeroth-order beam of the first order optical beam reflected by the first order side mirror 18 and a first-order beam of the zeroth order optical beam reflected by the zeroth-order side mirror 16, and optical detector elements 22A, 22B and 22C for photoelectrically converting the combined beam splitted by the beam splitter 20, respectively. Herein, the respective elements described above except for the scale constitute an optical detector.
In FIG. 10, polarizers 24, 26 which are inserted into optical paths of the zeroth and first-order optical beams, respectively, have directions of polarizations thereof intersecting perpendicularly to each other, and hence no interference fringe is formed on and around the optical detector 22A which is to receive the central one among the aforementioned three optical beams which are yielded as described above by splitting the combined optical beam into the aforementioned three optical beams. Therefore, a simple additive sum signal, not an interference fringe, is incident upon the optical detector element 22A. The signal is here used as a reference signal Vr.
Additionally, an analyzer 28B, which serves to produce an interference fringe, is disposed just before the optical detector element 22B, which then generates a phase A detection signal .phi.A which would be produced owing the interference fringe.
Further, a quarter wave plate 30 and an analyzer 28C are disposed just before the optical detector element 22C, which then generates a phase B detection signal .phi.B different in its phase by 90.degree. from the phase A detection signal .phi.A.
An incident angle .theta. of the laser beam 14 and a diffraction angle .phi. of the first order beam satisfy a relationship: EQU d(sin .theta.+sin .phi.)=.lambda. (1)
In such a grating interference type displacement meter, an optical grating of an 1 micro meter pitch or less can be achieved by manufacturing the diffraction grating 10A by holography for example, thereby assuring resolution of 0.01 micro meter.
However, when the glass surface of the scale 10 including the diffraction grating 10A formed thereon has bad flatness, in the transmission type grating interference type displacement meter as shown in FIG. 10, for example, angles of refraction of the zeroth-and first-order beams are changed and hence those optical beams are deflected as indicated by the arrow A in FIG. 11 (when the flatness of the lower surface of the scale is bad). As a result, directions of propagation of the two optical beams incident upon the optical detector elements 22B, 22C and inclined each other, and wave surfaces of the beams intersecting perpendicularly to these directions exhibit a pattern synthesized into a fringe shape, preventing a uniform interference pattern from being produced between the beams over the whole surface across a cross section on which the beams are superimposed. Accordingly, in such a transmission type grating interference type displacement meter, the flatness of the scale must be kept 5 micro meter/100 mm or less, and further no signal might be detected if the directions of optical axes would be inclined owing to any other factor.
On the contrary, in a reflection type grating interference type displacement meter in which a light source and a detector system are disposed together on one side of a reflection type scale, the light source and the detector system may be disposed on the one side of the scale, so that the reflection type one is suitable for a built-in type scale such as a separate type one. In such a reflection type grating interference type displacement meter, however, diffraction of a reflected light is used, so that displacement of an optical path originating from any inclination of the scale and insufficient flatness of the same is severer than the aforementioned transmission type is, requiring more accurate mounting and adjusting operations, which are difficult in execution.